1. Field of the Invention
The invention relates to a high pressure cleaning device, especially for cleaning with pressurized water, commonly referred to as a high pressure cleaner.
2. Description of Related Art
High pressure cleaning devices of the type in question have been known for a long time (see, e.g., German Patent 38 37 579). In many high pressure cleaning devices, the pump housing is separated from the valve pistol with the cleaning lance, the valve pistol being connected to the housing by a high pressure line several meters long. The housing, then, is stationary during operation, while the operator, with the valve pistol and the connected cleaning lance in hand, can work in an action radius determined by the length of the high pressure line. High pressure cleaning devices are also known that are made in one piece, in which, i.e., the valve pistol is integrated in the housing and the cleaning lance is connected to the housing. Those are so-called manual units, that are intended mostly for a relatively low pressure range up to 90 bar.
In a high pressure cleaning device of the type in question, generally, a bypass valve arrangement is present, also referred to as an unloader. The unloader is used to relieve the valve pistol of pressure when the valve pistol is closed. While working with such a high pressure cleaning device, one would like to leave the driving motor of the pump continue running during work pauses. So that the pump, on its output side, does not then work against the closed valve in the valve pistol, a bypass line back to the intake side of the pump is opened by the bypass valve arrangement, so that the pump circulates a finite, relatively small volume of cleaning fluid, which generally is only a few liters. The bypass valve arrangement known from the above-explained prior art reacts to the flow, and a flow unloader is involved. On the other hand, there are also spring-loaded bypass valve arrangements that are opened against the pressure of a spring whose spring resistance can be adjusted externally.
In large high pressure cleaning devices for high flow rates and/or high pressures, the materials used in the pump and in the connected lines are high-quality, expensive, and also heat resistant. Especially in smaller high pressure cleaning devices with lower flow rates, increasingly plastic parts are used, which is possible when modern plastics are used in view of the pressures occurring. However, when the pump continues to run for a prolonged time with a closed valve pistol, the temperature of the circulating cleaning fluid rises sharply because of the internal friction in the circuit. Thus, the manufacturers of high pressure cleaning devices in which this difficulty can occur specify in their operating instructions that the pump, during work pauses, is not to continue to run longer than a few minutes. Otherwise, damage, especially to the plastic parts of the pump and the lines, is almost a certainty.
The above-explained problem is known and a solution proposed in German Utility Model 93 15 960. For this purpose, a safety plug made of a material that becomes soft at a certain threshold temperature is attached to the pump intake line between the pump and a back-pressure valve which blocks the supply of water to the pump intake line when the pistol is closed. If the temperature of the cleaning fluid circulating in the circuit reaches the threshold temperature, the plug becomes soft and is pressed out of the pump housing. Water flows away abruptly out of the pump intake line, causing the back pressure to drop allowing fresh water to flow into the intake side of the pump after it, so that damage to the pump is avoided. But, the result is that the high pressure cleaning device is henceforth incapable of operating until overheating plug is replaced.
It is known in the art to provide, on a valve pistol for a high pressure cleaning device, a leakage flow passage that guarantees, at low temperatures, i.e., temperatures below the freezing point of water, a small leakage flow via the valve pistol, so that the valve pistol and the high pressure line do not freeze up (published German Patent Application 32 09 902). Such valve pistols are intended for stationary high pressure cleaning devices, as they are used at gas stations and in the commercial arena. For the small-unit arena, to which the present invention is directed, such valve pistols have not been used up to now, since there "frost protection" is not a problem. Structurally, the leakage flow passages in these freeze-preventing valve pistols are generally configured so they can be closed manually, so that the leakage flow can be turned off when temperatures are above the freezing point (between spring and fall each year).
Another valve pistol with a leakage flow passage for protection against freeze-up (German Utility Model 92 12 797) is configured to open the leakage flow passage only at very low pressure at the inlet side of the valve pistol. Such a valve pistol is suitable absolutely only for large, stationary high pressure cleaning-devices that provide a regular turning off of the high pressure cleaning device with the pump, and thus, the occurrence of low pressure (water-supply pressure). When the pump is running, and thus has higher pressure, the valve system of such a valve pistol is necessarily always closed, since otherwise, after opening once, it always remains open. When the pump is running, such a valve pistol allows no leakage flow, even with the use of an unloader.